1. Field of the Invention
The invention relates to the separation of gases by pressure swing adsorption processes and systems. More particularly, it relates to dual high purity gas product recovery therefrom.
2. Description of the Prior Art
Pressure swing adsorption (PSA) processes and systems are employed in a wide variety of industrial applications to produce high purity gas streams. In such processing, a feed gas mixture containing a more readily adsorbable component and a less readily adsorbable component are commonly passed to an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The bed is thereafter depressurized to a lower desorption pressure for desorption of the more readily adsorbable component from the adsorbent material and its removal from the bed prior to the introduction of additional quantities of the feed gas mixture to the bed as cyclic adsorption-desorption operations are continued in the bed. Such PSA processing is commonly carried out in multi-bed systems, with each bed undergoing the desired PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in each other bed in the system.
PSA systems are typically used in industrial applications to produce a single product stream from a given feed gas supply. For highly important air separation purposes, PSA systems achieve the desired separation of oxygen and nitrogen because of the greater selectivity of the adsorbent employed for either oxygen or nitrogen. The adsorptive capacity of any particular adsorbent material increases at higher pressure levels and decreases at lower pressures. In PSA processes and systems for the production of high purity oxygen product, the adsorbent employed may be one having a greater selectivity for either the desired oxygen product or for nitrogen. In systems in which the adsorbent employed is a nitrogen selective material, such as zeolitic molecular sieves, product oxygen is produced as the less readily adsorbable component removed from the bed during the adsorption step at the upper adsorption pressure. When oxygen is the desired product in systems employing an oxygen selective material, such as carbon molecular sieves, product oxygen is produced as the more readily adsorbable component upon the depressurization of the adsorbent bed to its lower desorption pressure. In PSA processes and systems in which nitrogen is the desired product, similar effects will pertain depending on whether the PSA system employs an oxygen or a nitrogen selective adsorbent.
Those skilled in the art will appreciate that PSA systems inherently can not completely separate any given feed stream component from the other components of the feed stream. In general, the PSA separation produces a product gas stream that contains a high percentage of one component together with a small amount of the remaining components. The other stream removed from the PSA system, i.e. the waste stream, will contain all of the incoming feed stream components. The fact that the adsorption system does not completely separate any component of the incoming feed stream from the other components is often the reason why a so-called waste stream exists in PSA processing. Quite frequently, this non-product waste stream does not contain a sufficiently high percentage of any given component to be of use in practical commercial operations. Therefore, this stream is of no significant value to the end user of the gas separation operation.
In the commercially important PSA-air separation technology, it is nevertheless desirable to recover the most prominent component of the waste stream, whether oxygen or nitrogen, as a separate high purity gas stream. Such recovery would serve to enhance the technical and economic feasibility of employing PSA operations in an ever-increasing field of industrially significant applications.
It is an object of the invention therefore, to provide a dual product PSA air separation process and system.
It is another object of the invention to provide a PSA air separation process and system capable of producing either oxygen or nitrogen product, together with a high purity stream comprising the most prominent component of the waste stream therefrom.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.